1. Field of the Invention
This invention relates generally to the field of seismic exploration and more particularly to an improved apparatus for imparting vibrational energy to the earth wherein selected combinations of compressional and shear wave energy can be generated simulataneously.
2. Description of the Prior Art
Simultaneous generation of compressional and shear seismic waves utilizing vibrating masses is taught by K. H. Waters in U.S. Pat. No. 4,321,981 entitled "Combination Shear Wave and Compressional Wave Seismic Energy Vibrator" issued on Mar. 30, 1982. Waters employs two or more vibrators whose vibrational axes are oriented along opposing non-vertical paths. The patented apparatus produces selectively proportioned simultaneous compressional and shear waves by controlling the relative phases of the two vibrating masses with respect to their axial positions at any moment. A limitation of the patented Waters apparatus is its inability to generate and record shear waves of both SH and SV type in a single operation. To record both such types of shear waves Waters first positions the base plate through which energy is coupled to the earth together with the vibrators themselves so that the vibrational axes are aligned transversely to the line of seismic survey extending between the vibrators and remotely positioned geophones. This enables recordation of P waves and SH type shear waves only. After operating the vibrators the base plate must be repositioned for a second operation so that the vibrational axes are aligned in the direction of the line of survey, thereby enabling recordation of P waves and SV type shear waves. Since information concerning both SH and SV type shear waves is desirable in order to obtain maximum information about sub-surface rock properties, it is advantageous to be able to measure and record both types of shear waves simultaneously, rather than sequentially.
A method and apparatus which provides simultaneous compressional and shear waves of both SH and SV type utilizing but a single vibrating mass is taught in commonly assigned U.S. patent application Ser. No. 792,531 filed Oct. 29, 1985 in the name of Tom Airhart and entitled "Full Wave Field Vibratory Seismic Source". The apparatus described in the subject patent application couples a vibrating mass to a ground engaging base plate so as to enable pivotal rotation of the vibrating mass about two mutually perpendicular rotational axes. In this way the vibratory axis of the vibrating mass may be made to coincide with any preselected vector path. This vector path can then be oriented with respect to a remotely located three element orthogonal geophone so as to enable simultaneous generation and detection of compressional and shear SH and SV waves. The relative amplitudes of such waves are varied by selecting any desired combination of tilt and azimuth for the vibratory axis. The vibrating mass is linked to the base plate through two separately rotatable shafts. In order to avoid attenuation or distortion of the resultant signal, the mechanical linkage through such shafts must be substantially free of misalignment and built to close tolerances.